Contract upgraded without my permission

Ive been a MTN customer for 18 yrs but Im so dissappointed that last month on the 14 July my contract no. *** and *** was upgraded without me being consulted. When I phoned MTN they told me it was upgraded by Mondo and they just transfered me to Mondo. I think since I have contract with MTN is their duty and responsibility to protect me from such things. Its over a month now and I cant get assistance on this matter because MTN refers me to Mondo and Mondo refers me to MTN and Im expected to pay the contact that I didnt approve the upgrade for. Im so dissappointed that Im considering cancelling all my MTN contracts because tgey cant just give out information to people we do t jnow